Wash the celery stalks and chop them roughly, peel and roughly chop the onion and celeriac. Melt the butter in a deep pan, tip in the vegetables and slowly cook for around twenty minutes, until soft. Pour in the stock, put in the bay leaf and bring to the boil, then lower the heat to a simmer and leave for around 30 minutes, stirring occasionally. Put the soup through a blender or food processor till really smooth. Check for seasoning (be aware that the cheese will be salty when added) and ladle into warm soup bowls. Crumble the cheese into the soup at the table and stir till partially melted.